1. Field of the Invention
The present invention relates to a high magnification zoom lens, and more particularly, it relates to a compact lightweight zoom lens that is approximately 75 degrees in shooting angle of view at a wide end, approximately F/3 to F/4 in F-number at the wide end, approximately F/5 to F/6 at a tele end, and approximately 6 to 7 in zoom ratio, and that is suitable for a single-lens reflex camera, a video camera, an electronic still camera, and the like,
2. Prior Art
In the prior art, there have been proposed a variety of zoom lenses such as high zoom ratio 4, 5 and 6-element lenses. However, such highly multi-element lenses are advantageous in view of correcting aberrations, but they tend to lead to an adverse effect of cost increase due to an increased number of components including a cam barrel. The whole lenses also tend to disadvantageously be larger. Instead, a reduced-element lens such as a 2-element lens encounters a difficulty in attaining both enhancing a zoom ratio and down-sizing because of its inherent lens property.
As to an improvement of a high zoom ratio lens in emphatically down-sized and weight-reduced design, a 4-element zoom lens is suitably implemented, with four of the elements having their respective refractive indices of positive, negative, positive, and positive levels in order from the closest to a subject, which is equivalent to those disclosed in Japanese Patent Laid-Open No. H8-211290 and Japanese Patent Laid-Open No. H9-5629, and is also equivalent to a product, Zoom Lens 71D (18 to 200 mm in focal length and F/3.8 to F/5.6 in F-number) available from TAMRON Incorporated.
Although a trend of down-sizing the high zoom ratio lenses has been drastic and rapid in recent years, improvements in the prior art are still yet bulky and heavy for practical use, compared with ordinary standard zoom lenses of approximately 28 to 105 mm in focal length and rough1 y F/3.5 to F/4.5 in F-number. In addition to that, insufficiently reduced dimensions of the prior art high zoom ratio lenses cause not only imbalance with more greatly down-sized and weight-reduced bodies of single-lens reflex cameras but also poor portability.
One of factors of the high zoom ratio lenses""staying larger than the standard zoom lenses is that a displacement of each of the elements comprised of a plurality of lenses is increased to attain a higher zoom ratio, and a subsequent variation in aberration is also increased, which, in turn, causes a difficulty in correcting the aberration in any point of focal range. To overcome such a disadvantage, several approaches have been attempted, including ways of reducing a refractive power of each element comprised of lenses to correct aberration, increasing the number of lenses in each element to correct aberration without reducing the refractive power of the element, configuring aspheric surface to correct aberration, and so forth.
However, the ways other than that using an aspheric geometry unavoidably make the whole lens dimensions larger. As with the way of using the aspheric geometry, however, simply increasing the number of surfaces causes further problems of a performance reduction due to a poor surface precision and of an increase in cost for a metal mold. For instance, the previously mentioned Zoom Lens 71D is comprised of 16 pieces of lenses, creating 2 of the aspheric surfaces, and this model is 81.5 mm in full lens length at a wide end and xcfx8672 mm in filter diameter, being 6 mm or more larger in full length and 10 mm or more greater in filter diameter than the above mentioned standard zoom lenses.
The zoom lenses as disclosed in Japanese Patent Laid-Open Nos. H8-211290 and H9-5629 are, when compared with the model 71D, almost the same in effective diameter of a front lens although having some variation in full lens length at a wide end, and thus, similar to the model 71D, these lenses cannot be categorized in the standard zoom lenses because of their insufficiently reduced dimensions.
The present invention is made, allowing for the above mentioned disadvantages in the prior art high zoom ratio lenses, and accordingly, it is an object of the present invention to provide a compact and lightweight high zoom ratio lens which is configured in 4-element zoom format with a deployment of positive, negative, positive, and positive lens elements in order, being approximately 75 degree in shooting angle of view at a wide end, approximately F/3 to F/4 in F-number at the wide end, and approximately F/5 to F/6 in F-number at a tele end so as to implement an enhanced high zoom ratio of approximately 6 to 7 and which is yet as large as standard zoom lenses (classified in those which are 28 to 105 mm in focal length and F/3.5 to F/4.5 in F-number).
A zoom lens according to the present invention includes first to fourth elements each comprised of a plurality of lenses, the elements having respective refractive indices of positive, negative, positive, and positive levels in order from the closest to a subject. In zooming from a wide-angle end to a telephoto end, the first and second elements of lenses have a space (air) enlarged therebetween, the second and third elements of lenses have a space (air) narrowed therebetween, and the third and fourth elements of lenses have space (air-filled) narrowed therebetween while the first, third, and fourth elements of lenses move toward the subject, and the second element of lenses reciprocally moves along an optical axis. In such a zoom lens, only the second element of lenses is moved for focusing while the conditions described as follows are satisfied:
0.065 less than xcfx86T/|xcfx862| less than 0.085xe2x80x83xe2x80x83(1)
0.35 less than xcfx86T/xcfx861 less than 0.55xe2x80x83xe2x80x83(2)
0.25 less than xcfx86T/xcfx864 less than 0.35xe2x80x83xe2x80x83(3)
0.75 less than |xcex22T|0.95xe2x80x83xe2x80x83(4)
where xcfx86T is a refractive power of the whole system at the telephoto end, xcfx862 is a refractive power of the second element of lenses, xcfx861 is a refractive power of the first element of lenses, xcfx864 is a refractive power of the fourth element of lenses, and xcex22T is an imaging power of the second element of lenses at the telephoto end (xcex22T less than 0).
The best mode of the present invention includes embodiments as described below.
The lenses of the third element are an aperture stop, a dual-sided convex positive lens, a positive meniscus lens having a convex surface faced toward the subject, and a negative lens which are all deployed in order from the closest to the subject, and the aperture stop is moved for zooming along with other lenses. The dual-sided convex positive lens has its one side configured in aspheric surface facing toward the subject, and such an aspheric geometry gives a property that any point in the edges farther from the center of the lens becomes greater in positive refractive power.
Additionally, the zoom lens according to the present invention satisfies the following condition:
xe2x88x920.05 less than 1/xcex23w less than 0 less than 1/xcex23T less than 0.30 xe2x80x83xe2x80x83(5)
where xcex23w is an imaging power of the third element of lenses at the wide-angle end, and xcex23T is an imaging power of the third element of lenses at the telephoto end.
Moreover, the zoom lens according to the present invention satisfies the following condition:
DWENP less than 28xe2x80x83xe2x80x83(6)
where DWENP is a distance from an apex of one side of the first element of lenses facing toward the subject at the wide-angle end to the center of an entrance pupil.
Furthermore, the zoom lens according to the present invention satisfies the conditions as follows:
0.22 less than |xcex22W| less than 0.3xe2x80x83xe2x80x83(7)
e0 less than 5xe2x80x83xe2x80x83(8)
h1+e0xc3x97tan xcex1W+fW/(2xc3x97FW) less than 25xe2x80x83xe2x80x83(9)
where xcex22W is an imaging power of the second element of lenses at the wide-angle end (xcex22W less than 0), e0 is a distance from the apex of one side of the first element of lenses facing toward the subject to a front principal point of the first element, h1 is a level at which extensions of principal rays incident at a half-angle of view come across a frontal principal plane of the first element of lenses at the wide end, which is expressed by an equation of computation in relation with paraxial rays, as follows:
h1=e1xc3x97e2xc3x97tan xcex1Wxc3x97(1/e1+1/e2xe2x88x92xcfx862)/((1xe2x88x92e1xc3x97xcfx861)xc3x97(1xe2x88x92e2xc3x97xcfx862)xe2x88x92e2xc3x97xcfx861)
where e1 is a distance between primary points of the first and second elements of lenses, which is expressed by an equation
e1=(xcfx861xc3x97xcfx862xe2x88x92xcfx861/xcex22W)/(xcfx861xc3x97xcfx862),
and e2 is a distance between primary points of the second element of lenses and aperture stop, which is expressed by an equation
e2=(1xe2x88x92hSTPxc3x97FW/fWxe2x88x92xcfx861xc3x97e1)xc3x97xcex22W/xcfx861
where hSTP is an open radius of the aperture stop, xcex1W is a half-angle of view at the wide-angle end, fW is a focal length of the whole system at the wide end, and FW is a F-number at the wide end.